1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a system and method for voltage conversion using a switching power converter with variations of a switch control signal pulse widths at low power demand levels.
2. Description of the Related Art
Many devices utilize electrical power to operate. Power is initially supplied by a power source, such as a public utility company, and power sources generally provide a steady state input voltage. However, the voltage levels utilized by various devices may differ from the steady state input voltage provided by the power source. For example, light emitting diode (LED) based lighting systems, typically operate from voltage levels that differ from voltage level supplied by a public utility company. To accommodate the difference between the voltage from the power source and the voltage utilized by the device, power converters are connected between the power source and the device to convert a supply voltage level from an alternating current (AC) power source to, for example, another AC power source having a voltage level different than the supply voltage level. Power converters can also convert AC power into direct (DC) power and DC power into AC power.
Switching power converters represent one example of a type of power converter. A switching power converter utilizes switching and energy storage technology to convert an input voltage into an output voltage suitable for use by a particular device connected to the switching power converter.
FIG. 1 depicts a power control system 100, which includes a switching power converter 102. Voltage source 101 supplies an AC input “mains” voltage Vmains to a full, diode bridge rectifier 103. The voltage source 101 is, for example, a public utility, and the AC mains voltage Vmains is, for example, a 60 Hz/120 V mains voltage in the United States of America or a 50 Hz/230 V mains voltage in Europe. The rectifier 103 rectifies the input mains voltage Vmains. The rectifier 103 rectifies the input mains voltage Vmains and supplies a rectified, time-varying, primary supply voltage VX to the switching power converter. The switching power converter 102 provides approximately constant voltage power to load 112 while maintaining a resistive input characteristic to voltage source 101. Providing approximately constant voltage power to load 112 while maintaining an approximately resistive input characteristic to voltage source 101 is referred to as power factor correction (PFC). Thus, a power factor corrected switching power converter 102 is controlled so that an input current iL to the switching power converter 102 varies in approximate proportion to the AC mains voltage Vmains.
PFC and output voltage controller 114 controls the conductivity of switch mode switch 108 so as to provide power factor correction and to regulate the output voltage VC of switching power converter 102. The PFC and output voltage controller 114 attempts to control the inductor current iL so that the average inductor current iL is linearly and directly proportional to the primary supply voltage VX. A proportionality constant relates the inductor current iL to the primary supply voltage VX, and the proportionality constant is adjusted to regulate the voltage to load 112. The PFC and output voltage controller 114 supplies a pulse width modulated (PWM) switch control signal CS0 to control the conductivity of switch 108. In at least one embodiment, switch 108 is a field effect transistor (FET), and switch control signal CS0 is the gate voltage of switch 108. The values of the pulse width and duty cycle of switch control signal CS0 depend on at least two signals, namely, the primary supply voltage VX and the capacitor voltage/output voltage VC. Output voltage VC is also commonly referred to as a “link voltage”. Current control loop 119 provides current iRTN to PFC and output voltage controller 114 to allow PFC and output voltage controller 114 to adjust an average iL current to equal a target iL current.
Capacitor 106 supplies stored energy to load 112 when diode 111 is reverse biased and when the primary supply voltage VX is below the RMS value of the input mains. The value of capacitor 106 is a matter of design choice and, in at least one embodiment, is sufficiently large so as to maintain a substantially constant output voltage VC, as established by a PFC and output voltage controller 114. A typical value for capacitor 106, when used with a 400 V output voltage VC, is 1 microfarad per watt of maximum output power supplied via switching power converter 102. The output voltage VC remains at a substantially constant target value during constant load conditions with ripple at the frequency of primary supply voltage VX. However, as load conditions change, the output voltage VC changes. The PFC and output voltage controller 114 responds to the changes in voltage VC by adjusting the switch control signal CS0 to return the output voltage VC to the target value. In at least one embodiment, the PFC and output voltage controller 114 includes a small capacitor 115 to filter any high frequency signals from the primary supply voltage VX.
The switching power converter 102 incurs switching losses each time switch 108 switches between nonconductive and conductive states due to parasitic impedances. The parasitic impedances include a parasitic capacitance 132 across switch 108. During each period TT of switching switch control signal CS0, energy is used to, for example, charge parasitic capacitance 132. Thus, switching power converter 102 incurs switching losses during each period TT of switch control signal CS0.
PFC and output voltage controller 114 controls the process of switching power converter 102 so that a desired amount of energy is transferred to capacitor 106. The desired amount of energy depends upon the voltage and current requirements of load 112. To determine the amount of energy demand of load 112, the PFC and output voltage controller 114 includes a compensator 128. An input voltage control loop 116 provides a sample of primary supply voltage VX to PFC and output voltage controller 114. Compensator 128 determines a difference between a reference voltage VREF, which indicates a target voltage for output voltage VC, and the actual output voltage VC sensed from node 122 and received as feedback from voltage loop 118. The compensator 128 generally utilizes technology, such as proportional integral (PI) type control, to respond to differences in the output voltage VC relative to the reference voltage VREF. The PI control processes the error so that the PFC and output voltage controller 114 smoothly adjusts the output voltage VC to avoid causing rapid fluctuations in the output voltage VC in response to small error signals. The compensator 128 provides an output signal to the pulse width modulator (PWM) 130 to cause the PWM 130 to generate a switch control signal CS0 that drives switch 108.
PFC and output voltage controller 114 modulates the conductivity of PFC switch 108. The primary supply voltage VX is, in at least one embodiment, a rectified sine wave. To regulate the amount of energy transferred and maintain a power factor close to one, PFC and output voltage controller 114 varies the period TT of switch control signal CS0 so that the inductor current iL (also referred to as the ‘input current’) tracks changes in primary supply voltage VX and holds the output voltage VC constant. As the primary supply voltage VX increases from phase angle 0° to phase angle 90°, PFC and output voltage controller 114 increases the period TT of switch control signal CS0, and as the primary supply voltage VX decreases, PFC and output voltage controller 114 decreases the period of switch control signal CS0.
Time T2 represents a flyback time of inductor 110 that occurs when switch 108 is nonconductive and the diode 111 is conductive. In at least one embodiment, the value of inductor 110 is a matter of design choice. Inductor 110 can be any type of magnetic component including a transformer. In at least one embodiment, the value of inductor 110 is chosen to store sufficient power transferred from voltage source 101 when switch 108 conducts in order to transfer energy to capacitor 106 when switch 108 is non-conductive to maintain a desired output voltage VC.
The inductor current iL ramps ‘up’ during time T1 when the switch 108 conducts, i.e. is “ON”. The inductor current iL ramps down during flyback time T2 when switch 108 is nonconductive, i.e. is “OFF”, and supplies inductor current iL through diode 111 to recharge capacitor 106. Discontinuous conduction mode (DCM) occurs when the inductor current iL reaches 0 during the period TT of switch control signal CS0. Continuous conduction mode (CCM) occurs when the inductor current iL is greater than 0 during the entire period TT.
The PFC and output voltage controller 114 sets a target current that tracks the primary supply voltage VX. When the inductor current iL reaches the target current during the pulse width T1, the switch control signal CS0 opens switch 108, and inductor current iL decreases to zero during flyback time T2. An average inductor current iL tracks the primary supply voltage VX, thus, providing power factor correction. Prodić, Compensator Design and Stability Assessment for Fast Voltage Loops of Power Factor Correction Rectifiers, IEEE Transactions on Power Electronics, Vol. 12, No. 5, September 1007, pp. 1719-1729 (referred to herein as “Prodić”), describes an example of PFC and output voltage controller 114.
PFC and output voltage controller 114 updates the switch control signal CS0 at a frequency much greater than the frequency of input voltage VX. The frequency of input voltage VX is generally 50-60 Hz. The frequency 1/TT of switch control signal CS0 is, for example, at or below 130 kHz to avoid significant switching inefficiencies.
FIG. 2 depicts a human audible sensitivity versus audible frequency band graph 200. The audible sensitivity plot 202 indicates the relationship between sound frequencies and human sensitivity to the sounds. References to “humans” refer to typical humans. Generally, humans can hear sounds having frequencies ranging from about 20 Hz to about 20 kHz. Thus, the audible frequency band is defined as approximately 20 Hz to 20 kHz. Human sensitivity to sound increases as the frequency increases from 20 Hz to about 1 kHz. At about 1 kHz, human sensitivity to sounds begins to decline as the sound frequency increases. Human sensitivity declines to 0 to sounds above about 20 kHz. In other words, sounds above about 20 kHz are generally inaudible to humans.
Humans are particularly sensitive to tones, such as tone 204, with frequencies in the audible frequency band. Human audible tones are repetitive sounds with a fundamental frequency in the audible frequency band. Humans are less sensitive to non-tonal, or noise-like sounds.
Referring to FIGS. 1 and 2, all magnetic circuit components of power control system 100 are small speakers. Physical forces on the wires and magnetic circuit components of power control system 100 cause the wires and components to vibrate at the switching frequency of switch control signal CS0. Switching frequencies below 20 kHz can produce audible tones that are generally deemed undesirable. Since the power demand by load 112 tends to change very little over a short period of time, such as 2-4 seconds, the frequency of switch control signal CS0 would remain approximately constant and produce an audible tone for switching frequencies less than 20 kHz.
Lower switching frequencies of switch control signal CS0 are generally desirable. High voltage transistors, diodes, and inductors often have significant parasitic capacitances that make high frequency switching of switch 108 inefficient. Additionally, high frequency switching increases electromagnetic interference (EMI) of power control system 100. Although lower switching frequencies are desirable, power control system 100 typically avoids switching frequencies below 20 kHz to avoid generating audible tones. Thus, the production of tones by power control system 100 limits the switching frequency range of switch 108.